Oxygen anion conductivity

The reaction rates of the thermal reduction and the reoxidation by CO2 are increased by high oxygen anion conductivity and high surface areas. Oxygen anion conductivity is a function of temperature, crystal structure, and defects. Because cycling results in stoichiometric gas-solid reactions, the gas-solid interface can be a crucial parameter depending on the reaction conditions. Whether gas-solid, intraparticle mass transfer, or surface chemical processes are rate-limiting is primarily determined by the reaction temperature and gas flow rates. 

Dense membranes have been used to feed hydrogen for hydrogenation reactions.Improved yields have been observed, due not to the above kinetics reasons, but attributed to the better availability of the active available on the membrane surface. Most dense membrane use has been to feed oxygen. Early studies considered silver membranes, but cost and low permeation rates did not favor these. More recent work has used solid oxide electrolytes as membranes. Initially, investigators used yittria- and calcia-stabilized zirconias (YSZ or CSZ), which had reasonable oxygen anion conductivity. Their low electron conductivity dictated the use of an external circuit, as shown in Figure 4(a). 

Tsai et a/. also approached the problem of increasing O2 flux and stability. Their approach was to balance the substitutions on the A and B sites of the ABO3 perovskite. Stability is strongly influenced by a stable BO3 skeletal sublattice, and the choice of Fe with a mild Co substitution gives stability with reasonable oxygen anion conductivity. Then different amounts and types of aliovalent cations (Ca, Sr ", Ba " ) were partially substituted for La in a LaFeo.8Coo.203 s perovskite framework to attain higher electron conductivity. 

With zeoUtes it is possible to compensate charging by heating the sample in situ to temperatures where either proton, cation or oxygen anion conductivity causes a complete breakdown of the insulating properties of the sample [7]. Inert sample holders, a well-outgassed sample and a very inert vacuiun environment are required in addition to a field-free heating device for the successful application of this technique. It is Umited to thermally stable zeolite samples, as temperatures of about 650 K are required for efficient compensation. 

SOFCs, SOECs, and SOERs all consist of three parts an electrolyte, an anode, and a cathode. The electrolyte is an oxygen anion conducting but electrically insulating solid oxide. This dense ceramic layer also acts as a barrier to prevent gas mixing between the two gas chambers. If we take the SOFC as an example. Fig. 1, the cathode is in contact with air, while the anode is in contact with fuel. The difference in oxygen partial pressure (pO-2)... 

In addition to providing a mechanism for oxygen anion conduction, these oxygen vacancies can act as intrinsic electronic donors, with electronic conductivity occurring via a hopping mechanism between B and B sites. 

All fuel cells have three components, anode, electrolyte, and cathode, as shown in Figure 38.3b. The anode in the SOFC is responsible for electrical chemical oxidation of fuels, the cathode is responsible for electrochemical reduction of O2 to 0 , and the electrolyte is responsible for the transport of 0 from cathode to anode. Both electrodes, that is, anode and cathode, must possess electrical and oxygen anion conductivities as well as catalytic activities. The most widely used and extensively studied fuel cells use Ni/... 

Borchert, H. and Baerns, M. The effect of oxygen-anion conductivity of metal oxide-doped lanthanum oxide catalysts on hydrocarbon selectivity in the oxidative coupling of methane. J. Catal 1997,168, 315-320. 

Figure 12. Dependence of oxygen-anion conductivity on CaO content at 1073 K (references see text).

Borchert, H. Baemsy, M. The Effect of oxygen-Anion Conductivity of Metal-Oxide Doped Lanthanum Oxide Catalysts on Hydrocarbon Selectivity in the Oxidative Coupling of Methane, y. Catal. 1997, 168,315-320. 

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